Passive sampling method for high-resolution concentration and isotopic composition of dissolved methane in Arctic lakes

Abstract

Lakes are important sources of methane (CH4), accounting for 6-16% of natural global CH4 emissions annually. Vertical spatial resolution of CH4 sampling in the water column of lakes is limited because of physical dimensions of sampling devices and sampling-induced mixing. To improve pool-size estimates of carbon, diffusive flux calculations for CH4, and detailed mechanistic understanding of CH4 dynamics in lakes, more precise vertical profiles of dissolved gases through the water column are vital. We investigate the use of commercially available passive diffusion bags (PDBs) to obtain highly resolved (0.5-m increments) vertical profiles of dissolved CH4 concentrations and isotopic composition (δ13 ). Laboratory experiments determined PDB equilibration time was between 5 d and 9 d for both CH4 concentration and δ13 . During July 2013, PDBs were deployed at intervals of 0.5-m depth for 5 d in two lakes located<5 km from the southwestern margin of the Greenland ice cap. Data from PDB samples were compared with samples collected using a submersible, electric pump. In both lakes, PDB and pump samples yielded vertical profiles for CH4 concentration and δ13 that were in close agreement (CH4 concentration R2=0.998, p<0.001; δ13 R2=0.970, p<0.001). Overall agreement between PDB and pump samples verifies the applicability of the PDB approach to dissolved CH4 sampling. High-vertical resolution sampling of dissolved gas concentrations and isotopic composition using PDBs can provide more accurate estimates of pool sizes and diffusive fluxes that advance understanding of production, consumption, and emission processes controlling trace gas dynamics in lacustrine environments under warming climate.